In recent years, a display panel provided with a touch sensor has attracted attention. Examples of the touch sensor includes a resistive touch sensor, a capacitive touch sensor, and an optical touch sensor which are different in operation principle. An object (e.g., a pen or a finger) touches a display device, whereby data can be input to the display device.
As an example of a device including such an optical touch sensor, a display device having an image capturing function, which is provided with a contact type area sensor that captures an image, is given (e.g., see Patent Document 1).